1- The germans were developing their own weapon.
2- The americans needed to show the world, and by world I mean the USSR, their new nuclear power.
After the war, the soviets and the americans started a game of "hungry hungry hippo" on the german scientists, who held the rocket technology developed for the V1. Yadda-yadda-yadda, Yuri Gagarin and Neil Armstrong.
Even now, 60 years later, you can still smell burned flesh in some parts of Hiroshima.